Photoinduced Bragg gratings are passive components with considerable potential in future telecommunications systems. In essence the gratings are lengths of optical waveguide, such as optical fiber, in which periodic variations of the refractive index have been induced. These periodic variations act as a Bragg grating, and selectively reflect light having a wavelength of twice the spacing. Such gratings can be used to filter, to define laser cavities and as components in multiplexers and demultiplexers.
Photoinduced Bragg gratings have been made in a variety of ways. An early approach was to form a reflecting surface on a short length of germanium glass optical fiber and to transmit a strong infra-red laser beam down the fiber to set up an interference pattern. Index perturbations occur at the maximum intensities. See U.S. Pat. No. 4,474,427 issued to Kenneth O. Hill et al, which is incorporated herein by reference. A second approach is to direct two interfering beams of ultraviolet radiation through the cladding of an optical fiber to form an interference pattern along a germanium-doped glass core. See, for example, U.S. Pat. No. 4,725,110 issued to Glenn et al. which is incorporated herein by reference. Another technique is to subject periodic regions of a fiber core to ultraviolet radiation, as through an amplitude mask. See U.S. Pat. No. 5,104,209 issued to K. O. Hill et al. which is incorporated herein by reference. And yet another approach uses a phase mask. See U.S. Pat. No. 5,327,515 issued to D. Z. Anderson et al. which is incorporated herein by reference.
The present applicants have discovered and disclosed in the aforementioned parent applications that the index-changing effect of actinic (UV) radiation can be enhanced by treating the glass with hydrogen or deuterium (Ser. No. 07/878,802 now issued as U.S. Pat. No. 5,287,427). They have further discovered that in hydrogen or deuterium-treated glass (hereinafter generically referred to as hydrogen-treated glass), the index of refraction can be increased not only by actinic radiation, but also by the application of heat (Ser. No. 08/056,329).
The present application is predicated upon the further discovery that in hydrogen treated glass, the alteration of the index of refraction can be substantially enhanced by simultaneously applying heat and actinic radiation.